As a result of the increased emphasis by Federal and State governments in combating air pollution, the coatings industry is expending considerable effort in eliminating or at least substantially minimizing the emission of solvent vapors to the atmosphere from coating compositions. As a part of this effort, the coatings industry has launched a major effort to develop aqueous or water-based coating compositions in which organic solvents have been completely eliminated or in which the organic solvents constitute only a very minor proportion of the total liquid medium.
In view of the excellent properties of solvent-based epoxy coating compositions for various coating applications, those in the coatings art have been extremely interested in developing aqueous coating compositions derived from epoxy resins. Prior attempts to develop such compositions involved reacting hydroxy carboxylic acids and epoxy compounds. However, in reacting such compounds, two types of reaction may result due to the chemical nature of the materials used. The hydroxyl groups of the hydroxy acid may react with the epoxide groups to form ether linkages, or the carboxyl group or groups of the acid may react with the epoxide to form ester groups. Both reactions may occur in an uncontrolled reaction to yield products having mixed ether or ester linkages to a non-predetermined degree. Such reaction with the epoxides and acids previously employed have not been tolerable since the ultimate products have not generally been suitable for any practical purpose.
Early efforts to solve these problems involved attempts to optimize the etherification portion of the reaction while minimizing the esterification portion of the reaction. (See, e.g., U.S. Pat. Nos. 3,404,018 and 3,410,773). In addition, attempts were made to utilize products containing both ester and ether linkages. (See, e.g., U.S. Pat. Nos. 3,707,526 and 3,792,112). However, these efforts were not particularly successful since compositions produced from these techniques exhibited significant disadvantages including poor cured film saponification resistance, low hydrolysis resistance, and lack of adequate package stability.
It has recently been proposed, as disclosed in U.S. Pat. No. 3,960,795, to prepare aqueous-based epoxy resins by a process which involves reacting an epoxy-containing organic material with a compound containing at least one phenolic hydroxyl group and a group hydrolyzable to a carboxyl group following which the resultant composition is hydrolyzed to generate carboxyl groups and then solubilized in known manner by neutralizing at least a portion of the carboxyl groups with a basic compound such as an alkali metal hydroxide or amine. In addition, it has also been proposed, as described in copending application Ser. No. 680,935, filed Apr. 28, 1976, commonly assigned to Applicant's assignee herein, to produce aqueous-based epoxy resins by a process which involves reacting an epoxy-containing organic material with a compound containing a mercaptan group and at least one group hydrolyzable to a carboxyl group, following which the resultant composition is hydrolyzed to generate carboxyl groups and then solubilized by neutralizing a portion of the carboxyl groups with a basic compound.
While the processes and products disclosed in the aforementioned patent and application are advantageous in many respects, they also exhibit significant disadvantages. Thus, for example, the processes described in the aforementioned patent and copending applications ordinarily require saponification, neutralization and filtration steps prior to neutralization with the base and solubilization in water. As will be evident, such processes, in view of the number of processing steps and procedures, can be time consuming and costly.